1. Field of the Invention
The present invention relates generally to communication systems and particularly to power control and a code division multiple access communication system.
2. Description of Related Art
The Federal Communications Commission (FCC) governs the use of the radio frequency (RF) spectrum, deciding which industry gets certain frequencies. Since the RF spectrum is limited, only a small portion of the spectrum can be assigned to each industry. The assigned spectrum, therefore, must be used efficiently in order to allow as many frequency users as possible to have access to the spectrum.
Multiple access modulation techniques are some of the most efficient techniques for utilizing the RF spectrum. Examples of such modulation techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). CDMA modulation employs a spread spectrum technique for the transmission of information. A spread spectrum system uses a modulation technique that spreads a transmitted signal over a wide frequency band. This frequency band is typically substantially wider than the minimum bandwidth required for transmitting the signal. The spread spectrum technique is accomplished by modulating each baseband data signal to be transmitted with a unique wideband spreading code. Using this technique, a signal having the bandwidth of only a few kilohertz can be spread over a bandwidth of more than a megahertz.
A form of frequency diversity is obtained by spreading the transmitted signal over a wide frequency range. Since only 200-300 kHz of a signal is typically affected by a frequency selected fade, the remaining spectrum of the transmitted signal is unaffected. A receiver that receives the spread spectrum signal, therefore, will be affected less by the fade condition. In addition, such a system has good performance in cases where interference may occupy a narrow-band.
In a CDMA-type radiotelephone system, multiple signals are transmitted simultaneously at the same frequency. A particular receiver then determines which signal was intended for that receiver by a unique spreading code in the signal. The signals at that frequency, without the particular spreading code intended for the particular receiver, appear to be noise to that receiver and are ignored.
Because code division multiple access networks employ a system in which all transmissions occur in the same frequency band, it is well known that it is important to transmit at the lowest possible power that allows for the delivery of a communication signal at a certain level of accuracy or grade of service criteria. The reason that it is important for base stations to transmit to mobile stations with a minimal level of power and, on the reverse link, for mobile stations to transmit to base stations with minimal amount of power, is that each transmission adds to the noise level for all other receivers. In addition, if the per user power on the forward link is minimized there is more power available for other users, thereby increasing the capacity of the system. Similarly on the reverse link, if less power is used, apart from the interference benefits mentioned above, the mobile station can extend its battery life and/or range of transmission.
Many different approaches have been developed to reduce power transmission levels of the base stations and mobile stations. For example, systems have been implemented in which a mobile station initially transmits at an estimated power level that is required to successfully deliver a communications signal. Thereafter, the transmissions between the mobile station and the base station are decreased in power until the signal to noise ratio or error rates fall between acceptable levels. Then, they are increased or decreased so as to transmit at a power level that delivers communication signals either at a threshold power level or having a threshold error rate. For example, if measured signal to noise ratio falls below a certain signal to noise ratio threshold, then power transmission levels are increased a notch. On the other hand, if frame or bit error rates exceed a defined threshold, then the power transmission level is ultimately increased to reduce the frame or bit error rate by first increasing the signal to noise ratio threshold.
Allowing quality of service of the call to degrade will reduce transmission power levels, which reduces noise and interference to other users. However, this is unacceptable from a user's perspective. Allowing too good of a quality of service, or improving quality of service beyond a certain point, on the other hand, does little to improve the user's perception of quality, but greatly increases transmission power levels, which, in turn, increases noise and interference to other users, causing their required power levels to increase in response. Ultimately, system throughput capacity is degraded.
While it is important to reduce the transmission power levels without excessively degrading communications, there is an ever present need to continue to find ways to reduce noise and interference among the transceivers while maintaining a certain level of system robustness or efficiency.